Bhat et al.—Late Triassic freshwater sharks from India
Table 3. Principal component coefficient of the first four axes. Variables
PC 1
Mediodistal length (MDL) Labiolingual width (LLW) Apicobasal height (ABH)
Height of principal cusp (PCH) 0.251 Eigenvalue % variance
0.817 0.275 0.439
92.565 1.582
–0.179 0.848 0.201 0.086 5.025
233
PC 2 PC 3 –0.456
PC 4
–0.387 0.029 1.696
–0.251 0.876 0.141
–0.244 0.354
–0.261 0.864 0.012 0.713
principal cusp. Because all of the variables were measured in the same unit (mm), the variance-covariance matrix is used for analyses (Hammer et al., 2001). As the first three principal components account for 99.29% of the total variance (Table 3), PC 4 is discarded. PC 1 is mainly a size axis, as shown by the high and positive coefficients of the four variables. On PC 2 or the first shape axis, high positive value is seen for crown heights (ABH and PCH), which load opposite to the crown length and width (MDL and LLW). On the other hand, LLW shows high positive value and loads opposite to other crown-proportion variables on PC 3. Principal component scores are plotted on PC 1 and PC 2
(Fig. 9.1), and PC 2 and PC 3 (Fig. 9.2) to show the scatter of the hybodont teeth examined. Although distinct clustering of the taxa is seen, Lissodus duffini and Parvodus tikiensis (Prasad et al., 2008) show close occupation of morphospaces with that of Pristrisodus tikiensis (Fig. 9.1). However, Parvodus rugianus shows considerable overlapping of convex hull polygon with that of Parvodus tikiensis and Pristrisodus n. gen., (Fig. 9.1), suggesting that all these taxa have similar size ranges. The other available lonchidiids examined (Parvodus curvidens, Lissodus minimus,and Jiaodontus [J. montaltissimus and J. vedenemus]) show distinctly separate occupation of zones in themorphospaces (Fig. 9.1, 9.2). Principal component scores plotted on PC2 and PC 3 (Fig. 9.2) show that the Indian taxa have overlapping of morphospaces with each other andwith that of Lissodus mimimus. Other taxa occupy separate and distinct morphospaces. CVA shows that 78% of the total teeth analyzed (N = 45)
have been correctly assigned (Table 4), whereas all the European forms show 100% correct assignment. The anterior teeth of L. duffini are 100% correctly assigned. Species of Jiaodontus show 67%–75% correct assignment of lateral teeth, whereas Parvodus tikiensis, as originally described by Prasad et al. (2008), show only 33% correctly assigned lateral teeth. Such a low percentage accounts for considerable overlap of the convex hull polygon of Parvodus tikiensis with that of Pristrisodus tikiensis (Fig. 10). On the other hand, the morpho- space of Lissodus duffini, although distinct, is close to the morphospace of Pristrisodus tikiensis. Such close clustering of the morphospaces of the Indian genera (Fig. 10) contrasts with that of other genera examined, in which the latter are distributed in distinct and separate morphospaces.
Discussion.—Fossil chondrichthyans are mostly described based on tooth morphology (e.g., Duffin, 1985; Shimada, 2002; Fischer et al., 2011; Johns et al., 2014), which has inherent problems because most of the teeth are found isolated and exhibit various forms of heterodonty (Shimada, 2005; White- nack and Gottfried, 2010; Cappetta, 2012). In the current work, application of PCA and CVA for hybodont teeth tests the validity of Pristrisodus n. gen., in comparison to other Indian, European, and Chinese taxa, and is represented graphically (Figs. 9, 10) by the occupation of different zones of morphos- paces (convex hull polygons). Although the sample size is small, in both the analyses, the specimens of Parvodus tikiensis and Lissodus duffini (Prasad et al., 2008) show either over- lapping of zones or close clustering with the morphospace of Pristrisodus tikiensis (Figs. 9, 10). These multivariate analyses thus corroborate the qualitative findings of the current study, where the specimens designated as P. tikiensis and L. duffini by Prasad et al. (2008) are found to be similar to the newly collected Tiki specimens, and are re-assigned to Pristrisodus n. gen. Moreover, loadings on different measured parameters in PCA (Table 3) show that crown height (ABH and PCH) loads opposite to crown length and width (MDL and LLW), and these crown proportions are key identification features for the differ- ent hybodont taxa analyzed.
Tooth histology
Description.—Two lateral teeth (morphotype IV, IITKGPP50 and morphotype III, IITKGPP29) were longitudinally and transversely sectioned, respectively, to reveal their tooth micro-morphology (Fig. 11). In both specimens, the crown has a thin (47 μm) outer enameloid layer overlying a thick dentine (Fig. 11.1). The latter is composed of an outer thick orthodentine (sensu Sire et al., 2009) and an inner narrow osteodentine (sensu Carlson, 1989). Although orthodentine usually com- prises an outer dense pallial dentine surrounding an inner layer of circumpulpar (Carlson, 1989), in Pristrisodus n. gen., the pallial dentine is restricted to the enameloid-dentine boundary and is hard to differentiate as in other hybodonts (Johnson, 2003). Profuse dentine tubules of two distinct types are seen in longitudinal view. The first type is coarse and situated towards the crown base (Fig. 11.2), whereas the second type is slender, feather-like, and found in the apical region of the crown (Fig. 11.3). In occlusal view (Fig. 11.4–11.6), a thick zone of ortho- dentine (285 µm along the vertical axis) overlies a narrow osteodentine (89 µm along the same vertical axis). The latter surrounds a small, central pulp cavity, which has a circular outline (diameter = 113 µm). The osteodentine is characterized by multiple, elliptical, and circular denteons that form a radiat- ing pattern surrounding the pulp cavity (Fig. 11.6).
Figure 8. Representative lonchidiid hybodonts, (1–2) Bahariyodon:(1) anterior tooth in lingual view, (2) lateral tooth in labial view; (3–5) Hylaeobatis:(3–4), symphyseal tooth in (3) occlusal, (4) lingual views, (5) lateral tooth in labial view; (6–7) Isanodus:(6) anterolateral tooth in lingual view, (7) posterior tooth in labial view; lateral tooth of (8–10) Vectiselachos:(8) occlusal, (9) labial, and (10) lingual views; (11–12), Diplolonchidion in (11) occlusal and (12, labial views; (13–15) Lissodus in (13) occlusal, (14) labial, and (15) lingual views; (16–18), Lonchidion in (16) occlusal, (17) labial, and (18) lingual views; (19–20), Parvodus in (19) labial and (20) lingual views; (21–24) Jiaodontus:(21–22) J. montaltissimus in (21) labial and (22) lingual views; (23–24) J. vedenemus in (23) labial and (24) lingual views; (25), Pristrisodus n. gen., (morphotype I) in labial view. Sources of information: Klug et al. (2010), Cappetta, (2012), and current study. Scale bars represent 1mm, except for 5mm (3–5), 500 µm(21−24).
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